TEM phase contrast imaging with image plane phase grating

ABSTRACT

Transmission microscopy imaging systems include a mask and/or other modulator situated to encode image beams, e.g., by deflecting the image beam with respect to the mask and/or sensor. The beam is modulated/masked either before or after transmission through a sample to induce a spatially and/or temporally encoded signal by modifying any of the beam/image components including the phase/coherence, intensity, or position of the beam at the sensor. For example, a mask can be placed/translated through the beam so that several masked beams are received by a sensor during a single sensor integration time. Images associated with multiple mask displacements are then used to reconstruct a video sequence using a compressive sensing method. Another example of masked modulation involves a mechanism for phase-retrieval, whereby the beam is modulated by a set of different masks in the image plane and each masked image is recorded in the diffraction plane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/075,031, filed Mar. 18, 2016, International PatentApplication No. PCT/US2016/023286, filed Mar. 18, 2016, and U.S. patentapplication Ser. No. 15/075,015, filed Mar. 18, 2016, all of which claimthe benefit of U.S. Provisional Patent Application No. 62/134,932, filedMar. 18, 2015, and U.S. Provisional Patent Application No. 62/148,619,filed Apr. 16, 2015. The entire disclosures of these applications areincorporated herein by reference.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with government support under grantsDE-FG02-03ER46057 and DE-AC05-76RL01830 awarded by the United StatesDepartment of Energy. The government has certain rights in theinvention.

BACKGROUND

In-situ transmission electron microscopy (TEM) can be a powerfulanalytical technique for enabling insight into the nature of materialsunder a broad range of environmental conditions. With the development ofa wide range of in-situ TEM stages and dedicated environmental TEM, itis possible to image materials under high-temperature, gas, and liquidconditions, as well as in other complex electrochemical, optical, andmechanical settings. In many of these applications, it may be desirableto capture the dynamic evolution of the microstructure with a highspatial and temporal resolution. While many developments in electronoptics and the design of in-situ cells have been made, leading tosignificant improvements in achievable resolution, there are still manychallenges associated with capturing dynamic processes with hightemporal resolution.

In-situ TEM video capture can be performed with charge-coupled device(CCD) cameras. High-performance commercially available CCD cameras havereadout rates in the range of a few tens of MB/s, which underappropriate binning conditions can provide a video acquisition period ofaround 30 ms. The introduction of direct detection cameras (DDCs) basedon CMOS technology can provide an order of magnitude increase in readoutrate, such that these cameras can be operated in the millisecond range.DDCs permit direct sensing of electrons without the use of ascintillator to provide electron-to-light conversion and can achievegreater sensitivity than that available using CCDs. While improvingtemporal resolution, DDCs can also enable electron dose reduction,another challenge for in-situ TEM imaging. A potential limitation isthat as frame rates increase, image read-out can become a challenge dueto the increased data rates.

SUMMARY

Imaging system comprise a modulator situated to receive a radiation beamand generate a mask-modulated radiation beam based on a relativedisplacement of a mask and a sample. A sensor is situated to receive themask-modulated radiation beam. In some examples, the mask comprises aplurality of transmissive and non-transmissive elements having apredetermined width and length, and the modulator is configured toproduce a displacement of the radiation image beam with respect to themask by at least the predetermined width or the predetermined length. Inother examples, the sensor is a sensor array. In further examples, theradiation beam is one of an X-ray beam, a gamma ray beam, a chargedparticle beam (CPB), an infrared beam, and an ultraviolet beam. In oneexample, the radiation beam is a CPB and the modulator is configured toproduce a displacement of a charged particle image beam with respect tothe mask by a distance proportional to a compression ratio of an imageintegrated by the sensor array. In some examples, a charged particlebeam source is situated to direct a charged particle beam to a sample soas to produce the charged particle image beam and the mask is positionedalong a charged particle beam path between the charged particle beamsource and the sample. In other embodiments, a receptacle is situated toretain the sample, wherein the mask is positioned along a chargedparticle beam path between the receptacle and the sensor array. Infurther examples, the modulator further comprises a translation stageconfigured to displace the mask with respect to the charged particleimage beam or piezoelectric positioner is coupled to the mask so as todisplace the mask along at least one axis. In some examples, a beamdeflector configured to scan the charged particle beam image withrespect to the mask in a predefined two-dimensional pattern.

Representative methods comprise spatiotemporally modulating a radiationimage beam based on a relative displacement of the radiation image beamand a mask and detecting the spatiotemporally modulated radiation imagebeam with a sensor array to generate a compressed video frame. Thecompressed video frame is decompressed and stored in a computer readablemedium. In some examples, the spatiotemporal modulation of the radiationimage beam is produced by moving the mask with respect to the radiationimage beam. According to some examples, the mask is translated in asingle direction during an integration time of the sensor array.

In other examples, a transmission electron beam microscope comprises abeam source configured to emit an electron beam and a sample receptaclefor receiving a sample so as to be irradiated by the electron beam andproduce an image beam. A modulator comprising a mask is situated toreceive the electron beam from the beam source or the image beam fromthe sample. The modulator is configured to produce a mask-modulatedimage beam based on relative displacements of the mask and the sample,the relative displacements typically associated with a predefinedperiodic scan pattern. An imaging system is coupled to receive themask-modulated image beam, wherein the imaging system is synchronizedwith the modulator so that at least a portion of the periodic scanpattern is followed during an integration period of the imaging system.In some examples, the modulator comprises a piezoelectric stage coupledto the mask to produce the relative displacement. In one example, themodulator comprises a beam deflector that moves the electron beam in arectangular pattern with respect to the mask during the integrationperiod of the imaging system.

Electron beam masks comprise a first plurality of pattern areasassociated with a first electron beam attenuation and defined by a firstelectron beam blocking material and a second plurality of pattern areasassociated with a second electron beam transmittance interspersed withthe first plurality of pattern areas. In some examples, the firstplurality of pattern areas associated with the first electron beamattenuation is defined in a common layer of the first electron beamblocking material and the second plurality of pattern areas associatedwith the second electron beam transmittance is defined as a plurality ofapertures in the common layer. In further examples, the second pluralityof pattern areas associated with the second electron beam transmittanceis defined as a plurality of thinned areas of the common layer. In stillother examples, the second plurality of pattern areas associated withthe second electron beam transmittance is defined in a second electronattenuating material. According to some examples, the first plurality ofpattern areas associated with a first electron beam attenuation isdefined by portions of the first electron beam blocking layer situatedon a surface of a substrate. In some cases, the second plurality ofpattern areas associated with the second electron beam transmittance isdefined by thinned portions of the substrate. In yet other examples, thepattern areas of the second plurality of pattern areas arepseudo-randomly interspersed with the pattern areas associated withfirst plurality of pattern areas. In typical examples, the firstelectron beam blocking material is glass, gold, platinum, iridium,osmium, nickel, and copper, tungsten, tantalum, uranium, lead, silver,as well as any other paramagnetic and nonmagnetic material, or anycombinations thereof and has a thickness of between 10 μm and 500 μm.

Representative methods comprise selecting a compressive sensing patternand defining the compressive sensing pattern in an electron beamblocking layer so as to establish pattern areas corresponding to a firstelectron beam transmittance and a second electron beam transmittance. Insome cases, the first electron beam transmittance is greater than thesecond electron beam transmittance. In one example, the pattern areasassociated with one of the first and second electron beam transmittancescorrespond to apertures defined in the electron beam blocking layer. Inother examples, the electron beam blocking layer is secured to asubstrate and the electron beam blocking layer is patterned as securedto the substrate. In some embodiments, the substrate is silicon nitride,silicon dioxide or silicon. In additional alternatives, at least one ofthe pattern areas associated with one of the first and second electronbeam transmittances is established in the substrate. In representativeexamples, the electron blocking material has a thickness correspondingto a standard attenuation at the first pattern areas or an attenuationdifference associated with the first and second electron beamtransmittances corresponds to a standard attenuation.

Methods of making an electron beam mask comprise exposing a photoresistlayer on a substrate so as to define photoresist areas corresponding toa first electron beam transmittance and a second electron beamtransmittance, wherein the first electron beam transmittance is greaterthan the second electron beam transmittance. The exposed photoresistlayer is developed so as to remove the photoresist layer at locations ona substrate surface associated with the second electron beamtransmittance and expose the substrate surface. An electron beamattenuating material is formed on at least the exposed portions of thesubstrate surface to produce mask elements associated with at least thesecond electron beam transmittance. An electron beam mask is formed byseparating the deposited material and the substrate. In otherembodiments, the electron beam attenuating material is deposited on thedeveloped photoresist layer, wherein the forming the electron beam maskincludes removing the attenuating material on the developed photoresistlayer by removing the photoresist layer. In some examples, thephotoresist areas associated with at least one of the first electronbeam transmittance and the second electron beam transmittance arerandomly or pseudo-randomly spaced or to satisfy a restricted isometryproperty associated with compressive sensing reconstruction.

In some examples, the mask areas associated with the first electron beamtransmittance cover 40% to 50% of an active mask area and definefeatures having dimensions of between 10 μm and 60 μm, the electron beamattenuating material is glass, gold, platinum, iridium, osmium, nickel,and copper, and combinations thereof. In typical embodiments, thephotoresist areas associated with the second electron beam transmittanceare rectangular areas having aspect ratios between one and six. In otherexamples, mask areas associated with the first electron beamtransmittance cover 1%, 2%, 5%, 10%, 20%, or 30% of an active mask area.In additional examples, a mask pattern that defines mask elements, isestablished and compressive sensing reconstruction with the mask patternis simulated. Based on the simulation, the photoresist areascorresponding to the first electron beam transmittance and the secondelectron beam transmittance are defined based on the mask pattern.

Electron beam masks comprise a perforated layer of an electron beamattenuating material having a pseudorandom placement of blocking areasinterspersed with voids in the perforated layer, wherein the perforatedlayer has a thickness of less than 100 μm. In some examples, the voidsor the blocking areas in the perforated layer satisfy a restrictedisometry property. In typical examples, the voids or the blocking areasare rectangular and have lengths and widths that are less than 20 μm. Instill other examples, the voids have a pseudorandom placement and theblocking areas have an electron beam transmittance of less than 40%.According to representative embodiments, the blocking areas arerectangular and have aspect ratios between one and four. In otherrepresentative examples, the blocking areas are gold having a thicknessof between 20 μm and 40 μm or the blocking areas are copper having athickness of between 70 μm and 90 μm. In further representativeembodiments, a frame secured to the perforated layer and the frame andthe perforated layer are portions of a common attenuating layer.

Methods of generating a pattern for an electron beam mask compriseinitializing pattern areas to have a first attenuation. A plurality ofnon-overlapping pattern areas are is configured to have a secondelectron beam attenuation that is different from the first electron beamattenuation, the plurality of non-overlapping pattern areas arranged toexhibit a restricted isometry property associated with compressivesensing reconstruction. The non-overlapping pattern areas arerectangular and are assigned lengths and widths, and have pseudo-randomoffsets. The pattern area assignments in a computer-readable storagedevice. In some examples, the first electron beam attenuation is greaterthan the second electron beam attenuation or the first electron beamattenuation is less than the second electron beam attenuation. Accordingto some examples, the rectangular pattern areas have aspect ratiosbetween one and four and the lengths and widths of the pattern areas arebetween 5 μm and 100 μm. In still further examples, the pattern areas ofeach of the plurality of pattern areas having the second CPB attenuationare separated by at least 5 μm and a fraction of the pattern areaoccupied by the plurality of pattern areas having the second CPBattenuation is between 40% and 50%.

According to some examples, a radiation beam is lens modulated based ona modulation of a characteristic of a lens that directs asample-modulated radiation beam to a sensor. The lens and samplemodulated radiation beam is detected with a sensor so as to produce adetected beam data series, and the detected beam data series isdecompressed. In some embodiments, the decompressed detected beam dataseries is stored in a computer readable medium. In other alternatives,the characteristic of the lens that is modulated is a lens focal length,a lens position along an optical axis, or a lens position transverse tothe optical axis. In other cases, the image beam is associated with asample area and the sensor is a single element sensor.

In other examples, imaging systems comprise a modulator situated togenerate a modulated image beam of a sample and a sensor situated toreceive and detect the modulated image beam at a diffraction plane. Aprocessor is coupled to the sensor and operable to produce a diffractionimage of the sample based on a plurality of detected modulated imagebeams detected at the diffraction plane. In some examples, thediffraction image is a combined phase and magnitude image. In typicalexamples, the modulator is coupled to modulate a beam focus, a beamdisplacement with respect to the sample and/or the sensor, or a beamangle of incidence at the sample or the sensor, or an interferencebetween beams. In other embodiments, the modulator includes at least onemask situated to receive a radiation beam and generate a mask-modulatedimage beam of the sample and the processor is operable to produce thediffraction image of the sample based on a plurality of detectedmask-modulated image beams detected at the diffraction plane. In somecases, the mask comprises a plurality of transmissive andnon-transmissive elements having a predetermined width and length, andthe modulator is operable to produce a displacement of a radiation imagebeam with respect to the mask by at least the predetermined width or thepredetermined length to produce the plurality of mask-modulated imagebeams.

In additional examples, methods comprise modulating a radiation imagebeam to generate at least one modulated image beam and detecting aplurality of modulated image beams with a sensor array at a diffractionplane to obtain a plurality of modulated diffraction images. The imagesof the plurality of modulated diffraction images are decompressed toobtain a diffraction image of a sample. In some examples, thediffraction image is stored in a computer readable medium and themodulation of the radiation image beam is produced by moving at leastone mask with respect to the radiation image beam. In someimplementations, the radiation beam is a charged particle beam (CPB) andthe modulation of the radiation image beam is applied by moving theradiation image beam with an electrostatic or magnetic beam deflector.In other examples, the radiation image beam is modulated to generate theat least one modulated image beam by modulating a beam focus, beamcurrent, beam position, sample position, or sample tilt. In typicalexamples, the diffraction image is a combined phase and magnitude image,a phase image, or a magnitude image.

Transmission electron beam microscopes comprise a beam source configuredto emit an electron beam and a sample receptacle for receiving a sampleso as to be irradiated by the electron beam and produce an image beam.At least one modulator is situated to receive the electron beam from thebeam source or the image beam from the sample, and is operable toproduce at least one modulated image beam. An imaging system is situatedto receive the at least one modulated image beam at a diffraction plane,and produce a magnitude, phase, or combined magnitude and phasediffraction image of the sample.

The foregoing and other features, and advantages of the disclosedtechnology will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a representative charged particle beam (CPB) imagingsystem.

FIG. 2 illustrates a representative method of charged particle beamimaging.

FIG. 3A illustrates a transmission charged particle beam microscopesystem that includes a moveable aperture.

FIG. 3B illustrates representative translations of the movable apertureof FIG. 3A.

FIG. 4A-4B illustrates an example controller and an associated timingdiagram for controlling a movable aperture.

FIGS. 5-6 illustrate CPB imaging systems that include beam deflectorsfor CPBs.

FIGS. 7A-7D illustrate representative displacements of a sensor arrayand CPB mask.

FIG. 7E illustrates a scan pattern associated with FIGS. 7A-7D.

FIG. 7F illustrates a representative mask.

FIG. 7G illustrates a sensor array.

FIG. 7H illustrates the mask of FIG. 7F with a frame.

FIG. 8 illustrates another representative scan pattern.

FIG. 9A illustrates an example method of generating a beam mask forcompressive sensing.

FIG. 9B illustrates another example method of generating a beam mask forcompressive sensing.

FIGS. 10A-10B illustrate simulated electron beam absorption andscattering characteristics for gold and copper.

FIGS. 11A-11B illustrate representative mask pattern.

FIG. 12 illustrates assignment of CPB transmittances to mask areas todefine a CPB mask.

FIG. 13 illustrates an example imaging system that includes a singleelement radiation sensor.

FIG. 14 illustrates an additional representative transmission imagingsystem showing alternative locations for a beam modulator.

FIG. 15 illustrates a transmission CPB diffraction imaging system.

FIG. 16 illustrates another representative mask pattern.

FIG. 17 is a block diagram illustrating a suitable computing environmentfor implementing some embodiments of the disclosed technology.

FIG. 18 illustrates a diffraction imaging system.

FIGS. 19-20 illustrate additional diffraction imaging systems that usemask modulation or modulation of other parameters of an imaging system.

DETAILED DESCRIPTION

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”Further, the term “coupled” does not exclude the presence ofintermediate elements between the coupled items.

The systems, apparatus, and methods described herein should not beconstrued as limiting in any way. Instead, the present disclosure isdirected toward all novel and non-obvious features and aspects of thevarious disclosed embodiments, alone and in various combinations andsub-combinations with one another. The disclosed systems, methods, andapparatus are not limited to any specific aspect or feature orcombinations thereof, nor do the disclosed systems, methods, andapparatus require that any one or more specific advantages be present orproblems be solved. Any theories of operation are to facilitateexplanation, but the disclosed systems, methods, and apparatus are notlimited to such theories of operation.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed systems, methods, and apparatus can be used in conjunctionwith other systems, methods, and apparatus. Additionally, thedescription sometimes uses terms like “produce” and “provide” todescribe the disclosed methods. These terms are high-level abstractionsof the actual operations that are performed. The actual operations thatcorrespond to these terms will vary depending on the particularimplementation and are readily discernible by one of ordinary skill inthe art.

In some examples, values, procedures, or apparatus' are referred to as“lowest”, “best”, “minimum,” or the like. It will be appreciated thatsuch descriptions are intended to indicate that a selection among manyused functional alternatives can be made, and such selections need notbe better, smaller, or otherwise preferable to other selections. In somecases, examples are described with reference to directions indicated as“above,” “below,” “upper,” “lower,” and the like. These terms are usedfor convenient description, but do not imply any particular spatialorientation.

The disclosed methods and apparatus generally pertain to particlebeam-based imaging systems, such as those associated with chargedparticles such as electrons or positively or negatively charged ions. Asused herein, “radiation” or “radiation beam” refers to charged particlebeams such as electron or ion beams, neutral particle beams such asneutron beams, electromagnetic radiation beams at wavelengths less than400 nm such as extreme ultraviolet, soft X-rays, hard X-rays, and gammaray wavelengths, or greater than infra-red wavelengths such asfar-infrared, microwave, millimeter wave, or longer wavelengths. Someradiation beams can be referred to as particle beams (e.g., ions,neutrons, electrons) while others can be referred to as high-energyelectromagnetic radiation beams. Propagating electromagnetic radiationat wavelengths between 400 nm and 700 nm is referred a visible beam.Visible and infra-red beams can also be used. As used herein, the term“image” or “image beam” can refer to a spatial variation in a particlebeam amplitude or phase, typically produced in a particle beam by aspecimen under investigation. For example, in transmission electronmicroscopy (TEM), an image beam is produced by directing an electronbeam to a specimen so that a transmitted beam has a spatially varyingamplitude or phase associated with the specimen. In some examples, acharged particle beam (CPB) such as an electron beam is scanned across aspecimen and spatial variations in the scanned beam correspond an imagebeam. The term “image” is also used to refer to a visible image such asobtained by displaying a CPB image on a display device or a fluorescentscreen. In addition, the term image can also refer to a detected imagecorresponding to numerical values associated with spatial variations inCPB beam intensity or phase as a function of position. For example,image beams can be detected to produce an array of CPB intensity valuesI(x,y) that are a function of transverse coordinates (x, y). Suchdetected images can be recorded and stored in a computer-readable mediumor transmitted as arrays, or in other image formats such as JPEG, TIFF,or other compressed or uncompressed formats. A sequence of beam imagesobtained at regular or irregular time intervals can be used to produce acorresponding sequence of recorded or stored images; such sequences arereferred to as videos, video signals, or video sequences. A single imageof a video sequence can also be referred to as a video frame. Imageshaving modulations associated with compressive sensing (CS) can bereferred to as compressed images or compressed video frames. In thedisclosed examples, array detectors such as CCDs or DDCs are typicallyused to detect particle beam images. Such detectors generally integratereceived particle beam intensity during a selectable frame duration. Asused herein, an integration time is a time period in which a sensorarray accumulates an image signal before capture and readout.

Because acquisition of image and video data is a common application ofthe disclosed methods and apparatus, the disclosed examples arefrequently described with reference to video or image capture, butsample diffraction patterns can also be acquired. Using the disclosedmasks to acquire multiple diffraction image can permit phase retrieval.

In typical examples, particle beams or other radiation beams aremodulated by transmission, reflection, scattering, or other interactionswith one or more beam masks. Such a beam mask is generally divided intoa plurality of mask elements arranged in an array, each of which can bearranged to produce a selected beam attenuation or phase change.Typically, the mask elements are arranged as a first set of maskelements and a second set of mask elements, wherein the first and secondsets of mask elements produce first and second attenuations and/or phasechanges or other modulations, respectively, wherein the first and secondattenuations and/or phase changes are different. Mask elements can havevarying dimensions and aspect ratios. For convenient description, maskareas are referred as transmissive or non-transmissive. As used herein,non-transmissive refers to transmittances of less than 25%, 10%, 5%, 1%,or 0.1%; transmissive refers to transmittances greater than 5%, 10%,25%, or 50%. Typically values are selected so that a ratio oftransmittances is at least 5:1, 10:1, 20:1, or 100:1. As used herein, areference or “standard” attenuation for a radiation beam at a selectedwavelength or of a selected particle type is an attenuation thatcorresponds to electron beam blocking provided by a 25 μm thickness ofgold.

In some cases, beam masks having first and second pattern areas with anattenuation difference that is about the same as the referenceattenuation are suitable. A particle beam that has been acted upon bysuch as mask can be referred to as a mask-modulated particle beam.Modulation for compressive sensing (CS) can be applied to particle beamswith masks or in other ways, and such particle beams generally referredto as modulated or encoded beams. Beam encoding can be applied bydirecting a particle beam to a mask and then to a specimen, or directingthe particle to the specimen and then to a mask. For some types ofradiation, spatial light modulators (SLMs) are available that permitmodeling radiation beams by varying characteristics of the SLM. Liquidcrystal based SLMs can be used for some wavelength ranges.

The term “signal” as used herein generally refers to a time-varyingparticle beam intensity or phase, or a time-varying electrical currentor voltage. Image signals are signals associated with images and videosequences, and are typically processed as a sequence of electricalimages represented as image currents or voltages.

In the disclosed examples, beam masks are situated at particularlocations for convenient illustration. For examples in which a beam maskis situated at a specimen or at a specimen image plane, such a beam maskcan generally be situated at other locations that are opticallyconjugate to the specimen. Similarly, in examples in which a mask orsensor is situated at a transform plane, such a mask or sensor can besituated at other conjugate locations. Conjugate planes can be providedwith one or more beam lenses that image such planes onto each other viaone or more intermediate planes.

In the disclosed examples, binary masks are used in which some maskpattern areas apply a first modulation and other mask pattern areasapply a second modulation. These modulations are conveniently describedas amplitude modulations but other types of modulations can be applied.In addition, masks having two, three, or more sets of pattern areas thatapply respective modulations can be used. For example, three sets ofpattern areas can apply first, second, and third modulationsrespectively. CS-based systems can use such modulations as well. Suchmasks can use different thicknesses or types of material or combinationsthereof to provide such different modulations. These differingmodulations can be between 0% and 100% for masks that provide amplitudemodulation. However, for convenient illustration, the examples aredescribed with reference to binary masks.

Beam modulation can also be provided by directing one or more opticalbeams (such laser beams at the same or different wavelengths) so as toscatter electrons from an electron beam so that the scattered electronsdo not reach a sensor or sensor array. In still other examples, opticalelements such as lenses (for particle beams or electromagneticradiation) can provide beam modulation by varying lens focal length orlens position along or transverse to an imaging system optical axis,beam angle of incidence, or other lens characteristics. Beam modulationcan also be provided by varying beam phase or coherence with an appliedelectromagnetic field. In addition, a beam source can be modulated. Insome examples, an array of emitters with independently modulatableemitters can be used to generate a spatially modulated beam.

Introduction

Compression of a signal can reduce an amount of data needed to representa video, provide increased temporal or spatial resolution in image orvideo data, permit reduced data acquisition times while maintainingsignal to noise ratio, or provide other signal analysis and acquisitionfeatures, typically based on the use of signal sparsity and incoherence.Compression can occur after videos, images, diffraction patterns, orother data have been captured and stored. However, as described herein,compression can occur during the acquisition of a video by usingcompressive sensing (CS). Compressive sensing combines sensing andcompression in one operation to increase the temporal resolution of anydetector (both CCDs and DDCs). Using CS, fewer total measurements may berequired, which, when applied to particle beam video capture, canpotentially increase the acquisition speed and reduce required particlebeam dose.

Using the approaches described herein, an effective frame rate of acamera can be increased by modulating an image prior to acquisition byan imaging sensor. As one example, modulation is applied by directing aparticle beam received from a sample (i.e., an image beam) to a particlebeam mask and then to an image sensor. The mask can be moved at a fixedor variable rate so that a sequence of images associated with differentmodulations or encodings by the mask are acquired and combined in asingle frame. Alternatively, a modulation can be applied to a particlebeam with a stationary mask, and the particle beam scanned over themask.

Upon acquisition of a sequence of images, the images can be decompressedas described below. In one example, a sequence of encoded TEM images isacquired, and a reconstructed (i.e., decompressed) image sequence can beused to estimate an electron beam dose associate with an onset of beamdamage.

Using CS, an image signal can be compressed at the time of measurementand accurately recovered at a later time with suitable decompressionusing hardware and/or software. In imaging applications, the compressioncan be applied spatially to reduce the number of pixels that need to bemeasured. This can lead to an increase in sensing speed, a decrease inthe amount of image data needed, and dose reduction, which can beespecially helpful in electron microscopy. In video applications, thetime dimension can be compressed. By compressing the sensed data intime, the total frame rate of a camera system can be increased by afactor associated with a compression ratio by integrating a sequence ofcoded images into a single frame.

In some respects, the CS methods and apparatus disclosed herein arecounter-intuitive to traditional signal acquisition techniques whichsample and then compress the sampled data. Traditional approaches aremotivated by the Nyquist-Shannon sampling theorem, which states that inorder to accurately reconstruct a signal, it must be sampled at afrequency at least twice the highest frequency present in the sample.For example, the sampling theorem states that for a band-limited signalthat is a sum of three sine waves with different frequencies (such as 1Hz, 9 Hz, and 64 Hz) and amplitudes, a sampling frequency of at least128 Hz is required to reconstruct the signal. Yet, in the frequencydomain, three samples are sufficient; the signal is said to be 3-sparseunder the Fourier basis. In CS, non-adaptive sensing measures signals ina basis that tends to make the signal as sparse as possible so that thenumber of measurements can be reduced below the Nyquist rate. Differentbases can be selected for CS approaches, such as Fourier modes orwavelets, but it is also possible to discover the basis from themeasurements.

CS can be implemented in a variety of ways. In one example, CS can beapplied to video signals by using a coding scheme called pixel-wiseflutter-shutter which selectively blocks one or more pixels on an imagesensor during an integration time. (In the following, boldface type isused to represent matrices.) A value Y_(ij) associated with a singlepixel of a measured image can be represented as:

${Y_{ij} = {\begin{bmatrix}A_{{ij}\; 1} & A_{{ij}\; 2} & \ldots & A_{ijL}\end{bmatrix}\begin{bmatrix}X_{{ij}\; 1} \\X_{{ij}\; 2} \\\vdots \\X_{ijL}\end{bmatrix}}},$wherein A_(ijf) are binary indicators of whether a mask element blocks apixel ij in a frame f, X is an image, and L is a number of frames(images). This representation can be simplified as Y_(ij)=Φ_(ij)x_(ij),wherein

$x_{ij} = \begin{bmatrix}X_{{ij}\; 1} \\X_{{ij}\; 2} \\\vdots \\X_{ijL}\end{bmatrix}$and Φ_(ij)=[A_(ij1) A_(ij2) . . . A_(ijL)]. A complete representation ofmask modulation can be provided as a block diagonal matrix Φ which isgiven by:Φ=diag(Φ₁₁,Φ₁₂, . . . ,Φ_(N) _(x) _(N) _(y) ).wherein the image size is N_(x)×N_(y) pixels. In some examples, imagesare divided into patches so the image data x_(i) in a mixture of factoranalyzers (MFA) model is of size based on the patch size. For patch sizeof 4, the image data points is of size 4×4×L.

Compressed measurements for CS-MFA can be obtained by spatially and/ortemporally modulating an image beam with a coded aperture mask. Forexample, an image beam can be directed to a coded aperture mask thatapplies a modulation to the image beam at a rate faster than a rate atwhich an image sensor produces images. Such a modulation providesacquired images based on multiple mask encodings. Modulation can beapplied in various ways. As one example, a mask is translated orotherwise subjected to a time-varying displacement using one or moretranslation stages, such as piezoelectric stages. Such stages can bemoved along one or more axes based on applied stage drive signal. If atriangular wave drive signal having up and down ramp portions is used,sets of encoded images can be obtained during the up ramp and the downramp; stage motion along orthogonal axes can be used. Differentpositions of the mask with respect to the image establish differentimage encodings, and different masks are not required to producedifferent encodings. Since a time-varying position of the mask can beobtained based on the applied stage drive signal, image encoding as afunction of time is known. A compression ratio of a modulated image canbe determined based on a range of motion of the mask. Effectively,moving n feature sizes or pixels of the mask can provide a compressionratio of n and n differently encoded images are combined in single framecaptured by an image sensor if a suitable image integration time isselected. To produce a CS modulated image, these n different images areto be integrated into a single frame.

As described herein, particle beam imaging systems using CS include amask such as a self-supported, single-piece mask that includes aplurality of mask regions that apply different modulations. For example,the mask regions can selectively apply different amplitude or phasemodulations. In convenient examples, such a mask for particle beamimaging using transmission includes mask regions that are substantiallynon-transmissive and mask regions that are substantially transmissive.Such a mask can be formed by patterning transmissive andnon-transmissive regions on a substrate, and an array of patternedregions removed from the substrate. For example, non-transmissiveregions can be formed using electroplating of materials that arenon-magnetic and have a high blocking power for charged particles, suchmaterials having large atomic numbers. The transmissive regions can bevoids or holes in the non-transmissive regions. Generally, any patternedarrangement of regions with differing attenuations can be used fortransmission particle beam imaging.

Example 1 Charged Particle Beam Imaging System

FIG. 1 illustrates a representative charged particle beam (CPB) imagingsystem 100. It should be noted that FIG. 1 and the other Figures hereinare provided for illustrative purposes and are not to proportion or toscale unless specifically noted as being to scale. The CPB imagingsystem 100 include a CPB column 105 for forming and transforming acharged particle beam that is directed toward a sensor array 170. Thecolumn 105 includes various CPB optical components, such as one or moreof beam sources, lenses, apertures, deflection systems, alignment coils,blanking plates, scintillators, and stigmators. As shown in FIG. 1, thecolumn 105 includes a beam source 110 that directs a beam toward samplethat is retained by a sample receptacle 120. A modulator 130 is situatedbetween the sample and the sensor array 170 so as to apply a modulationto a transmitted CPB image beam. The column 105 is generally coupled toa vacuum pump and one or more power supplies for operation at suitablepressures and to control and adjust CPB optical elements, but forconvenient illustration, these are not shown in FIG. 1. The CPB from thebeam source 110 is transformed as it passes through and interacts withthe various components of the column 105. For example, an image beamproduced by transmission of the CPB through the sample has a spatiallyvarying beam intensity or phase based on spatially varying CPBabsorption, reflection, deflection, and/or transmission in a sample.

In one embodiment, the CPB imaging system 100 is a transmission electronmicroscope (TEM) system and the beam source 110 generates an electronbeam. The beam source 110 can be oriented to irradiate an area of asample placed in the sample receptacle 120. Alternatively, a scannedbeam can be used such in a scanning TEM in which a small area beam isscanned across the sample, typically in a raster pattern.

The sample receptacle 120 can be positioned along a charged particlebeam path between the beam source 110 and the modulator 130. The samplereceptacle 120 can be used to hold a sample and can include calibrationcontrols to bring the region of interest into the path of the chargedparticle beam. For example, the calibration controls can include steppermotors, piezoelectric positioning stages, screw gearing, or othermechanical and/or electrical devices suitable for adjusting atranslation and/or rotation of the sample within the sample receptacle120. The sample receptacle 120 can include an airlock for enabling asample that is not under a vacuum to intersect with the charged particlebeam. For example, the sample can be placed within a dedicated gas cellholder that can be inserted into the sample receptacle 120. The cellholder can also include a heater so that the sample can be tested undera pressure and temperature that is different from the pressure andtemperature of the column 105. The cell holder can include a standardsize grid to place the sample or the sample can be self-supporting. Gridmaterials can be copper, molybdenum, gold, or platinum, for example.

The modulator 130 is situated to receive the image beam. In one example,the modulator 130 includes a mask 150 that includes a pattern of maskregions that are non-transmissive (or substantially attenuating) andtransmissive (substantially non-attenuating) that receives the imagebeam. The transmissive and non-transmissive regions of the mask can havea predetermined width and length. Each of the transmissive andnon-transmissive modulation regions can have a different predeterminedwidth and length. The width and length can be based on a variety offactors, such as a resolution of a photoresist used to create the mask150, a pixel size of the sensor array 170, properties of the materialforming the non-transmissive areas, and other factors.

The non-transmissive areas can be constructed of a material thatpartially or completely blocks charged particles for a given chargedparticle energy. The blocking can be caused by absorption or bybackscattering, for example. As a specific example, non-transmissiveareas can block over 90% or 95% of 300 kV electrons that are incident onthe non-transmissive areas. In one embodiment, an active area of themask 150 to which a CPB is directed is self-supported using only thestructure of the non-transmissive areas. In other words, the active areaof the mask 150 can include transmissive holes or voids andnon-transmissive areas that also from a support structure. A non-activemask area is an area that is not intended for CPB beam modulation butcan be provided as a support for the active area. In some cases thesupport is a perimeter region that forms a frame for the active maskarea.

The modulator 130 can be configured to produce a relative translation ordisplacement of an image beam with respect to the aperture mask 150. Forexample, the relative translation can cause an image beam modulation ata rate faster than an integration time or frame rate of the sensor array170. In particular, the modulator 130 can be configured to produce adisplacement of the image beam with respect to the aperture mask 150 byat least the predetermined width or the predetermined length of one ormore of the transmissive and non-transmissive modulation regions.

As one example, the relative translation or displacement can be causedby a stationary beam and a moving aperture. In particular, the modulator130 can include a translation or positioning stage 155 that can iscoupled to move the mask 150 with respect to the CPB. Alternatively,beam deflector 140 can deflect the image beam with respect to the mask150 and a beam deflector 160 can return the mask-modulated image beam tosubstantially its original propagation direction so as to be incident tothe sensor array 170 with little or no displacement due to this beamdeflection.

Typically, the stage 155 (and/or beam deflectors 140, 160) and thesensor array are coupled to a processor 180 for control and datatransfer. The processor can also include one or more computer-readablestorage devices that have processor-executable instructions fordecompressing compressed images to produce video sequences. Compressedand uncompressed image and video data (and control instructions anddata) can be stored in storage device 190.

Example 2 Charged Particle Beam CS Methods

FIG. 2 illustrates an example method 200 for CPB imaging using CS. Sucha method can be implemented using various CPB imaging systems such as,for example, the imaging system shown in FIG. 1. At 210, a chargedparticle (CP) image beam is formed by, for example, exposing a sample toa CPB such as an electron or ion beam. As a specific example, the CPBbeam can be transmitted through the sample to form the CP image beam.Variations in intensity or phase in the CP image beam are associatedwith differences in structure and/or composition of the sample. At 220,the image beam can be spatiotemporally modulated using displacement ofthe CP image beam with respect to a mask. The CP image beam can bemodulated in a variety of ways including by moving a mask with respectto the CP image beam and/or by moving or scanning the CP image beamrelative to the aperture. As one example, the beam is modulated bymechanically moving the mask using a translation or positioning stage.

At 230, the modulated image beam is detected to capture a compressedvideo frame using a sensor array that detects a varying intensity of themodulated beam. The modulated image beam can cover all of or a portionof a sensor array area to convert a time-varying intensity of themodulated image beam into a series of compressed video frames. Forexample, the modulated beam can be directly sensed by CMOS sensors of aDDC or indirectly sensed using a scintillator and a CCD array. Eachcompressed video frame can be collected during a sensor integration orexposure time.

At 240, the detected compressed video frame can be stored in a computerreadable medium. The compressed video frame can be stored in associationwith metadata about the video frame such as a timestamp, sequencenumber, or other information about relationships to other video frames,information about the experimental setup, and so forth. In someexamples, a timestamp can be used to determine mask position (and hence,mask modulation) for decoding. The computer readable medium can includeone or more of volatile memory, non-volatile memory, removable medium,non-removable medium, and/or any other medium which can be used to storeinformation in a non-transitory way and which can be accessed by aprocessor of the imaging system.

At 250, an uncompressed video frame is reconstructed from the compressedvideo frame using compressive sensing reconstruction. For example, aprocessor can execute a software reconstruction module that uses one ormore compressed video frames stored on the computer readable memory tocreate the uncompressed frame. At 260, the uncompressed frame is storedin a computer readable medium.

An integration or frame-grab of the modulated image beam with a sensorarray can be synchronized with the movement of the positioning stage.The encoded mask can be moved in a predetermined pattern in one or moredirections. However, the mechanical response of the positioning stagemay be more controllable when the stage moves in a linear directionalong a single axis. The amount and velocity of movement caused by thepositioning stage can be controlled based on various factors, such as amaximum range of the positioning stage (e.g., 500 μm), and/or a desiredrate of travel of a feature of the aperture relative to the sensorarray. For example, increasing the rate of travel may increase an amountof compression in the modulated beam.

As noted above, in another example, the image beam can be modulated byscanning or deflecting the beam (such as by using electron optics) withrespect to the encoding aperture. On the one hand, moving the image beammay enable a wider range of movement than moving the mask because theelectronics can be more responsive with fewer or no hysteresis effects.For example, charged beam or electron optics can be used to move theimage beam in more complicated patterns, such as along multiple axes. Onthe other hand, moving the image beam may cause a different pixel mixingat the sensor array compared to only moving the aperture. For example,moving the aperture may not cause the image to move relative to thesensor array. However, moving the image beam can cause the image to moveor be displaced relative to the sensor array. Thus, the same pixel ofthe sensor array can detect different portions of the image as the imageis moved. A reconstruction algorithm can potentially account for boththe displacement of the image and the modulation of the beam.Alternatively, the beam can be deflected both before and after theaperture so that the image is incident on the sensor without beingdisplaced. It should be noted that moving a sensor array with respect toa mask can have a similar effect as moving the mask with respect to thesensor array. However, the sensor array may have more mass and moreconnections to other components and so moving the aperture mask may bepreferable to moving the sensor array. For purposes of illustration, theexamples are described with reference to a stationary sensor array, butare readily adapted to a movable sensor array or a combination of maskand sensor array movement. In addition, in the examples, a mask istypically situated downstream of a sample, but can be situated prior tothe sample as well.

Example 3 Charged Particle Beam Microscopy System

A representative CPB microscopy system 300 using a stationary beam and amoving aperture is illustrated in FIG. 3. A CPB source 310 directs a CPB315 to a specimen 320 so as to produce an image beam 325. A positioningstage 355 is coupled to a mask 350 that is scanned with respect to theimage beam 325 to produce a mask-modulated image beam 340. A sensorarray 370 receives the mask-modulated image beam 340 and integrates fora time duration established by a controller 380, or otherwisedetermined. Detected images are stored in a memory 390. For continuousdetection of the modulated image beam, an integration time can be lessthan or equal to a frame time (a reciprocal of the frame rate). Forexample, one image can be collected while a previous captured image isscanned out of the sensor array 370.

The microscopy system 300 can include sensor arrays of various sizes andresolutions. For example the sensor array 370 can have an array 512×512,1024×1024, 2048×2048, 3K×4K, 5K×4K, pixels, or other numbers of pixels.Individual sensor elements of a sensor array can range in size from 10μm on a side to 2.5 μm on a side, for example. As described furtherbelow, an element or feature size defined on the mask 350 can beselected to be proportional to the size of individual sensor elements ofa sensor array. In some examples, one or more CPB optical elements arecontrolled to magnify or demagnify a CPB image based on mask elementdimensions. For example, mask element dimensions can be multiples ofindividual sensor element dimensions with or without magnification ordemagnification.

The frames or images collected by the sensor array 370 can be stored inthe storage device 390 that can include volatile memory (e.g.,registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flashmemory, etc.), or some combination of the two, accessible by the sensorarray 370 and the processor 380. The storage device 390 can includeremovable or non-removable components, and include magnetic disks,magnetic tapes or cassettes, CD-ROMs, DVDs, or any other medium whichcan be used to store information in a non-transitory way and which canbe accessed within the microscopy system 300. In addition to storing theimages collected by the sensor array 370, the storage device 390 canalso include software implementing one or more innovations describedherein, in the form of computer-executable instructions suitable forexecution by the processor 380. For example, the processor 380 canexecute software modules for accessing the compressed image datacollected by the sensor array 370 and reconstructing uncompressed imagesusing a compressive sensing reconstruction algorithm. The reconstructinguncompressed images can be stored in the storage device 390. As anotherexample, the processor 380 can execute software modules for controllingthe stage 355 and/or other components of the microscopy system 300.Typically the mask 350 is displaced along two orthogonal directions 356,357.

The mask 350 applies a spatial encoding to the image beam 325 based onan arrangement of transmissive and non-transmissive areas of the mask350. The non-transmissive areas of the mask 350 can occlude more of theimage beam 325 than the transmissive areas of the mask 350. As oneexample, the non-transmissive areas of the mask 350 can occlude greaterthan 90% of the image beam 325 and the transmissive areas of the mask350 can occlude less than 10% of the image beam 325. As the mask 350 isdisplaced by the stage 355, a different spatial encoding is applied tothe image beam 325 based on the position of the mask 350. The image beam325 is modulated so that beam modulation varies during an integrationtime associated with image or frame capture by the image sensor 370. Itshould be noted that the mask 350 generally is moved continuously duringan integration time (and not discretely). FIG. 3B illustrates threedifferent displacements of a mask 350 that are associated with scantimes T0-T3.

The mask 350 can be moved in a predefined periodic pattern so that theimage sensor 370 can capture multiple video frames. The image sensor 370is synchronized with the modulator 330 by the controller 380 so that therelative displacement associated with a predefined periodic scan patternis followed during a sensor integration period. For example, the sensorarray 370 can be synchronized with the modulator 330 so that a firstvideo frame (integrating multiple encodings) is captured by the imagesensor 370 as the mask 350 is moved in a first direction at a firstspeed, and a second video frame is captured by the image sensor 370 whenthe mask 350 is moved in a direction opposite of the first direction atthe first speed. The controller 380 can be used for synchronizationincluding signaling the positioning stage 355 and triggering the imagesensor 370.

Example 4 Image Acquisition and System Control

FIG. 4A illustrates an example controller 400 and FIG. 4B illustrates arepresentative timing diagram 450 for controlling mask displacement. Thecontroller 400 includes a computer 402 such as a desktop, laptop,tablet, handheld or other computer that is coupled to one or moredisplacement stages 408 so as control or receive a mask position. Thecomputer 402 includes a sensor controller 410 such as a CCD controllerthat is coupled to a sensor array 412. The computer 402 initiates imageacquisition by the sensor array 412 and receives, stores, and processesimage data received from the sensor array 412. The sensor array 412 iscoupled to a divide/delay circuit 414 so as to produce a control signalto initiate and terminate stage motion. The divide/delay circuit 414 iscoupled to a waveform generator 416 that can deliver a stage drivesignal to the displacement stages 408. In other examples, the computer402 initiates image acquisition and generates stage control signalswhich in some cases, require additional processing such asamplification. The computer 402 can execute computer-executableinstructions to communicate with and initiate and terminate actions ofthe sensor array 412 and the stage controller 406.

Referring to FIG. 4B, in some examples, image acquisition andintegration by the sensor array 412 is provided by coupling an integratesignal 451 to the sensor array 412. Typically, the integrate signal 451is asserted during an integration time period 452 when a modulated beamis being detected by the sensor array 412. The integrate signal 451 istypically de-asserted for a time period 453 between integration timeperiods. A frame period 454 is a time period between capture ofconsecutive compressed video frames by the sensor array 412. A framerate is the reciprocal of the frame period 454.

The sensor array 412 can provide a trigger out signal 460 that occurs inresponse to a beginning of an integration time period. For example, thetrigger out signal 460 can be a short pulse that occurs in response to abeginning of an integration time period. The trigger out signal 460 maybe delayed by a delay time 413 from the beginning of the integrationtime period due to delays caused by buffer circuits and other electroniccomponents. The trigger out signal 460 can be further delayed using thedivide/delay circuit 414 that provides a divide and delay signal 465 tothe waveform generator 416. A delay is established so that sensorintegration times are suitably aligned with a stage control signal 470so that movement of the aperture mask has a desired set of properties.For example, the delay 451 can be tuned so that a mask is moving at arelatively constant speed during a sensor integration period. Thewaveform generator 416 can generate the stage control signal 470 as atriangle wave at a frequency of the divide and delay signal 465. Thestage control signal 470 can drive the stage(s) 408 directly or throughthe stage controller 406. For example, the stage(s) 408 can bepiezoelectric positioners with multiple piezo stages. One piezo stagecan control the coding motion and two remaining stages can beincorporated for aligning the aperture mask to intersect the image beam.The stage control signal 470 can cause one of the piezo stages to movethe mask back and forth periodically along a linear direction. Forexample, the mask can be moved in one direction when the stage controlsignal 470 has a positive slope and in the opposite direction when thestage control signal 470 has a negative slope. The mask may move at amore constant velocity when the stage control signal 470 is farther froma trough and peak of the triangle wave. For example, the mask candecelerate, reverse direction, and then accelerate at each of thetroughs and peaks of the triangle wave. Thus, the integrate signal 451can be aligned with the stage control signal 470 such that imageintegration by the sensor array 412 is enabled during time periods inwhich the mask is moving at a more constant velocity (e.g., away fromthe troughs and peaks).

Example 5 Image Encoding with Scanned Beams

Additionally or alternatively, the relative translation or displacementof the charged particle beam image with respect to a mask can beobtained by moving or scanning the image beam. Referring to FIG. 5, abeam source 510 directs a particle beam 515 to a sample 520 to producean image beam 525 that is directed to a modulator 530. The modulator 530includes a beam deflector 540 and a mask 550. A deflection controller580 is coupled to the beam deflector 540 so as to sweep, scan, orotherwise direct the image beam 525 across the mask 550. As a result, amask-modulated image beam 535 is directed to a sensor array 570.Typically, the beam deflector 540 includes one or more deflection coilsor electrostatic deflectors that are controlled to scan the image beamin a predetermined pattern across the mask 550.

Modulation of an image beam as shown in FIG. 5 produces mask-modulatedimage beam similar to those produced with mask displacements. However,moving an image beam with respect to a mask can be simpler as mechanicalstages are not needed. Of course, mechanical scanning and beam scanningcan be used together, if preferred. Scan patterns can be periodic onedimensional or two dimensional patterns.

The scanning of the image beam 525 can be synchronized with imageintegration by the sensor array 570 using the controller 580. Forexample, a period of the beam scan pattern can be synchronized with anintegration period of the sensor array 570. As discussed above, theimage beam 525 is modulated at a rate faster than the sensor array framerate so that multiple mask-modulated images are acquired during a sensorarray integration time. In the example of FIG. 5, each modulated imagecan be scanned so as to be incident to the sensor array at a locationdifferent than without beam scanning. Thus, typically, detected imagedata associated with multiple sensor array elements is combined toproduce a compressed image or compressed video. By contrast, withmechanical scanning, each image area with an associated modulation isdirected to a fixed sensor area. Thus, image reconstruction with beamscanning can require preprocessing to obtain compressed images prior toreconstruction as done with mask-displacement based methods.

Example 5 Image Acquisition with Beam Scanning and Restoration

Image displacement at a sensor array in beam scanning can be avoided.Referring to FIG. 6, a CPB imaging system 600 includes a beam source 610that directs a CPB 615 through a sample 620 to produce an image beam 625that is directed to a modulator 630. The modulator 630 includes a firstbeam deflector 640 situated to deflect the image beam with respect to amask 650 to produce a mask-modulated image beam 635. A second beamdeflector 660 is situated to apply a deflection to the mask-modulatedbeam 635 so as to compensate beam deflection supplied by the first beamdeflector 640. As a result, the mask modulated beam 635 is received at asensor array 670 with suitable time-varying modulation by the mask 650but with minimal time varying beam displacement. Thus, sensor elementsof the sensor array 670 receive modulated beam portions associated witha fixed sample area. Modulation by the mask 650 is selected bydeflection controller 680 that is coupled to the first beam deflector640 and the second beam deflector 660. The controller 680 cansynchronize the first beam deflectors 640, the second beam deflector660, and image acquisition by the sensor array 670.

Example 6 Mask Scanning

FIGS. 7A-7F illustrate representative displacements of a mask 700 withrespect to a sensor array 705 having representative sensor elementsS(I,J), wherein I, J are integers that denote rows and columns. Forconvenience, the sensor array 705 includes eight rows (numbered 0-7) andeight columns (number 0-7), and a sensor element situated in an upperleft hand corner is identified as sensor element S(0,0). The mask 700 isdefined by an array of mask elements that selectively attenuate orotherwise modulate a CPB. For convenience, a modulation associated witha mask element (I,J) is referred to as M(I,J), wherein I, J are integersthat denote rows and columns of the mask. In the example of FIG. 7, themask 700 includes ten rows and ten columns, and an upper leftmost maskelement provides a modulation M(0,0).

FIGS. 7A-7D illustrate movement of the mask 700 with respect to thesensor array 705 to obtain a compressed image. The mask 700 is shown inFIG. 7F and the sensor 705 is shown in FIG. 7G. FIGS. 7A-7D showdifferent displacements 710, 720, 730, 740 of the mask 700 with respectto the sensor 705 and FIG. 7E illustrates a sequence in which thedisplacements of FIGS. 7A-7D are produced. These displacements areprovided in a single sensor integration time. For ease of illustration,only displacements corresponding to a whole sensor/mask elements areshown, but during integrations, there are fractional displacements. Asdiscussed above, the mask/sensor displacements 710, 720, 730, 740 can beprovided by translating the mask 700 with respect to the sensor array705 along one or more axes. In this way, image beam modulation can beobtained. In FIGS. 7A-7D, mask element spacing and size are selected tocorrespond to sensor element size and spacing, so that at certain timesas the mask 700 is scanned, a single mask element corresponds to asingle sensor element, but other sizes and spacings of mask elements andsensor elements can be used. A single mask element can correspond tomultiple sensor elements.

For convenient illustration, mask and sensor elements are shown in FIG.7A-7D without gaps. For example, the mask elements are shown as havingselected attenuations that extend to adjacent mask elements. In someexamples, transmissive apertures (or CPB blocking regions) associatedwith a mask element cover only a portion of an available mask area forthe mask element. The sensor array area 705 can be further divided intosubareas corresponding to mask element sizes and each mask element sizecan correspond to multiple sensor elements.

The amount of movement within an integration time of the sensor array705 can determine an amount of compression within a video frame capturedby the sensor array 705. In FIGS. 7A-7D, there are four distinctencodings at four different displacements during the integration time ofthe sensor array 705. As used herein, a distinct encoding is an encodingthat differs from an earlier or later encoding by at least one maskelement. The compression ratio can be proportional to the number ofdistinct encodings within the integration time of the sensor array 705,so the compression ratio is four in this example.

FIG. 7H is a further illustration of the mask 700, showing a frame 701that surrounds the active area of the mask, and can be formed of thesame material as the mask elements but for which patterning is notneeded.

FIG. 8 illustrates an example two-dimensional rectangular mask pattern800 that is scanned with respect to a sensor array 805 to create acompression ratio of eight for a compressed video frame. The maskpattern 800 applies eight encodings to an incident beam within a sensorintegration time, wherein each encoding is based on a displacement of atleast one mask element from other encodings and is thus different fromother encodings. In the example of FIG. 8, two vertical and twohorizontal displacements are used.

Example 7 Masks and Methods

Masks are typically defined so as to include single piece active areaswith enough mechanical structure to be self-supporting. In particularexamples, non-transmissive areas of a mask provide mechanical structureand the transmissive mask area are defined by voids or holes. Duringmanufacture, a substrate can be used for support and then removed orsplit from the mask elements. A mask can be fabricated in various wayssuch as by laser drilling, mechanical drilling, etching, andmicro-electroplating. Micro-electroplating can provide desirablecharacteristics such as providing a well-defined aperture features ofdimensions of between 5 μm and 20 μm, compatibility with materials thathave suitable structural stability and electron stopping power even inthin layers, enabling mask elements having a small aspect ratio betweentop and bottom, providing long-range periodicity and reasonable cost.Mask generally define pattern areas associated with two different beammodulation values such as beam attenuations, beam phases, or other beammodulations.

FIG. 9A illustrates an example method 900 of generating a mask for acharged particle beam system using compressive sensing. At 910,photoresist can be deposited over an area of a substrate. The substratecan be used as support structure for the aperture mask duringmanufacture of the aperture mask. The substrate can be glass or silicon,for example. In one embodiment, the substrate can be conductive or caninclude a monoatomic or thicker layer of a conducting material on asubstrate surface. A photoresist can be selected based on a variety offactors, such as a resolution or sensitivity of the photoresist, forexample. The photoresist can be applied in various ways to produce auniform thickness that is typically at least as thick as a CPBattenuating material to be deposited. In some examples, photoresistlayers of thickness greater than 30 μm or greater than 100 μm areformed. A photoresist layer can be formed by spin coating, spraycoating, or roller coating onto a substrate surface. The photoresistlayer can be cured by air drying and/or by curing at a temperatureexceeding room temperature, for example.

At 920, the photoresist is patterned to uncover or expose areas of thesubstrate corresponding to mask elements that are intended to beattenuating or non-transmissive to a CPB. In particular, the patternedphotoresist layer can define photoresist areas corresponding torectangular transmissive elements of a CPB mask and areas of exposedsubstrate corresponding to non-transmissive elements. A photoresistlayer can be patterned by exposure to a positive or negative image of anintended mask pattern so as to selectively expose areas of thephotoresist. The photoresist material can be a positive or negativephotoresist which exhibits increased or decreased solubility afterexposure. The photoresist is developed using a solvent or etchant thatremoves exposed or un-exposed areas, depending on whether a positive ornegative photoresist is used. If a positive photoresist layer is used,photoresist areas corresponding to non-transmissive mask areas areremoved by developing. Alternatively, if a negative photoresist layer isused, photoresist areas corresponding to the transmissive mask elementsare exposed, and the photoresist at non-transmissive areas of the maskcan removed.

At 930, a non-transmissive material is deposited over at least theexposed substrate areas. The non-transmissive material is depositedusing electroplating, sputtering, evaporation, or other process to athickness sufficient to produce a selected CPB attenuation. At 940, thepatterned photoresist layer is removed along with any non-transmissivematerial deposited on the photoresist layer, and the substrate isremoved from at least areas associated with mask elements that are to betransmissive. At 950, the mask is secured to a mechanical frame withglue, soldering, or other process. In some examples, a portion of thesubstrate to which photoresist is applied can be retained to serve as aframe.

Using electroplating for depositing a non-transmissive material maypermit mask elements to have straighter, sharper edges. Such edges mayimprove image quality in a mask-modulated image by reducing reflectionsfrom sides of non-transmissive mask elements. In an alternative method,a material (such as a metal) is deposited at a desired thickness on asubstrate and the resulting metal layer lithographically processed toremove the material at areas associated with transmissive mask elements.For example, a photoresist can be applied to a metal layer formed on asubstrate. The photoresist layer can be patterned and exposed so thatportions of the metal layer and the substrate associated withnon-transmissive mask elements are exposed and can be etched to removeor thin. In some examples, such etching can create conical holes due toisotropic etching of the substrate that can result in reduced imagequality due to reflections from the sides of the aperture mask.

Another method 970 of fabricating a radiation beam mask is illustratedin FIG. 9B. At 972, a photoresist layer is deposited on a beamattenuation layer, such as a gold layer that can be provided as a foillayer. For convenient processing, such beam attenuation layer can besecured to another substrate for simpler handling during processing. At974, the photoresist layer is exposed to a mask pattern, and at 976, thephotoresist is developed 976 so that portions of photoresist remain atmask areas that are to relatively more attenuating while at areas thatare to be relatively less attenuating, the beam attenuation layer isexposed. The exposed areas of the beam attenuation layer are wet etched,dry etched, or ion beam milled at 978 to reduce beam attenuation layerthickness, and in some examples, holes in the beam attenuation layer areformed. In some cases, directly etch or milling processes are preferred.At 980, the photoresist layer is removed, and the patterned beamattenuation layer can be secured to a frame.

In other alternatives, a beam attenuating layer is directly machined byion beam milling or other process without a photoresist protectivelayer. In some examples, a beam attenuation layer on a substrate isprocessed to define mask areas having different attenuations. Thesubstrate is selected to be substantially transparent to a radiationbeam, and typically has a transmittance of at least 10%, 25%, 50%, of75%. In representative examples, masks include patterned beamattenuation layers or portion=s thereof on silicon nitride, silicondioxide, or silicon layers of thicknesses between about 10 nm and 20 μm,typically between 10 nm and 40 nm. With sufficiently thin substrates,the beam attenuating layer need not be separated.

Example 8 Representative Materials

The desired thickness can be based on a variety of factors such as ablocking power of the non-transmissive material and a mechanicalstrength of the non-transmissive material. A relationship of thicknessand blocking power for two materials is described further below withrespect to FIGS. 10A-10B. A material for the non-transmissive areas canbe selected based on a variety of factors, such as cost, mechanicalstrength, magnetic properties, and blocking or stopping power.Properties of suitable aperture materials can include being non-magnetic(e.g., having a low relative permeability) and having a high blockingpower for electrons, such as having a high atomic number. By selectingnon-magnetic materials, images can exhibit less distortion than by usingmagnetic materials that can deflect the CPBs transmitted by the mask.High-Z materials also permit thinner layers and tend to reducereflections from the side-walls of transmissive areas. Suitable aperturematerials can include various transition metals having atomic numbers ina range of 72 to 80. Typical examples of suitable materials includeglass, gold, platinum, iridium, osmium, nickel, and copper, andcombinations thereof.

The thickness of attenuating material in non-transmissive mask elementscan be based on the material and a desired attenuation. As one example,the thickness can be between 30 μm and 200 μm, depending on thematerial. FIGS. 10A-10B show simulated electron beam transmission,backscatter, and absorption as a function of layer thickness for goldand copper, respectively. As shown in FIGS. 10A-10B, to obtain 95%blocking, thicknesses of gold and copper of 30 μm and 80 μm,respectively, are needed. To obtain 20% transmittance, a 20 μm goldlayer or a 58 μm copper layer is needed. In other examples,non-transmissive mask elements are defined by nickel in thicknessessimilar to those required for copper. A glass layer about 200 μm can beused to achieve about 5% transmittance.

In one embodiment, non-transmissive mask elements are defined by a goldlayer 20 μm to 40 μm thick, or a copper layer 60 μm to 90 μm. In someexamples, thinner layers are preferred to reduce reflections from sidesof the mask elements. Thicker layers provide superior mechanicalstrength. In some examples, a mask is mounted in a frame to provideadditional structural stability.

Example 9 Representative Masks

FIGS. 11A-11B illustrate representative masks 1100, 1100 in whichnon-transmissive areas are illustrated as dark areas and transmissiveareas are illustrated as light areas. As described above, thetransmissive areas can be holes, voids, or thinned regions of asubstrate or absorbing layer. Non-transmissive areas are provided with athicker layer of an absorbing layer than that associated withtransmissive areas, or a more absorbing material or a less thinnedsubstrate. In some examples, the transmissive areas are rectangularareas that are selected to be randomly or pseudo-randomly distributedacross an active area of a mask and can be selected so as to satisfy arestricted isometry property (RIP) associated with compressive sensingreconstruction, but mask patterns can be selected in other ways as well.Generally, a mask satisfying the RIP property can have a transmissivityof up to 50%. However, the transmissivity may be lower (such as 40-45%)to provide sufficient structure for a self-supporting aperture mask. Insome examples, the mask elements satisfy pseudo-random or randomarrangements are used.

Example 10 Mask Pattern Selection

The spatial arrangement of the transmissive and non-transmissive areasin a mask can be determined using a random or pseudorandom placement oftransmissive and non-transmissive elements constrained by suitabledesign rules. The design rules can include rules to maintain structuralsupport for the aperture, rules for a maximum non-transmissive area,rules for a minimum-size step area (e.g., a minimum sized area for arepeating pattern), rules for a maximum run-length, and/or rules for amaximum diagonal length. Repeated or periodic patterns of transmissiveand non-transmissive areas are generally disfavored as periodic patternscan create support or grid lines that always block some sensor elements.Patterns that are connected from one corner of a mask to an oppositecorner of a mask along a diagonal may have a tendency to flop as theaperture is moved, and tend to be associated with reduced imagingperformance and reduced mask lifetime. A random pattern of mask elementsexhibiting local connectivity may be preferred than a less randompattern over larger distances.

Various approaches can be used to generate a pattern for the mask. Insome cases, mask element dimensions are sufficiently large that a singlemask element maps to multiple sensor elements of a sensor array. Ineither approach, all mask elements can be initialized to benon-transmissive and then transmissive mask elements can be added to themask pattern to make the pattern more transmissive. Mask elementdimensions can be selected based on a sensitivity of the photoresistused to manufacture the aperture and/or desired optical properties ofthe aperture mask pattern. The feature size of the mask can specify thesmallest dimension for a transmissive and/or non-transmissive area. Thefeature size of the mask can be based on a size of one or more sensorsof the sensor array. In particular, the feature size of the mask can bea multiple of the sensor size, such as between three and four times thearea of a sensor of the sensor array. As a specific example, anindividual sensor can be a square that is 6.4 μm on a side, and thefeature size of the mask can be about 25 μm along one dimension so thata smallest transmissive area can overlay a 2×2 array of sensor elements.

A mask pattern can include a grid or webbing of non-transmissive areasthat can be used as a support structure for the mask. The transmissiveareas can be randomly binned or selected to be placed within regionsbounded by the grid-lines. Thus, there can be a random distribution ofON (binned) transmissive areas and OFF (not binned) non-transmissiveareas across the area of the aperture mask. The distribution of ON andOFF binned pixels can be selected to have a sparse random distributionthat satisfies the restricted isometry property for CS reconstruction.The support webbing can potentially reduce the overall transmissionthrough the mask which can reduce the resolution at the sensor arraydetector.

Prior to mask construction, a mask pattern is generated evaluated todetermine if appropriate for CS applications. Of course, a mask havingthe pattern can be used to make measurements, but CS simulation isgenerally preferred. Model signal data or image data is modulated withthe mask pattern, and the modulated data or image processed to produce afinal decompressed output. This output can be evaluated based on spatialor temporal resolution, probable sample dose, and signal-to-noise ratio.If satisfactory, the pattern can be used to define a physical mask.

In FIG. 12, non-transmissive mask regions are illustrate with shading;transmissive areas are shown without shading. A mapping to a sensorarray is illustrated by a grid 1221 within the area 1220. Initially, allportions of a representative area 1220 (or all regions of an entiremask) are selected to be non-transmissive. Mask regions within the area1220 are then grouped into clusters having various shapes and sizes. Forexample, transmissive mask elements can range in size from a minimumsize to a maximum size. As a specific example, a random width and arandom height between the minimum size and the maximum size can beselected. In one example, mask feature width is selected to correspondto a width of three sensors elements of a sensor array. A transmissiveelement (cluster) of minimum size is shown as mask area 1222. Sincetransmissive areas can have different widths and different heights,transmissive mask areas can have different aspect ratios. As anotherexample, a random aspect ratio and orientation can be selected for atransmissive mask region. As a specific example, transmissive maskregions (clusters) 1223 and 1224 have a common aspect ratio (4/5), butdifferent orientations. The mask region 1223 is oriented to have heightalong a Y-axis that is greater than a width along an X-axis; the maskregion 1223 is oriented to have a height along the Y-axis that is lessthan a width along the X-axis. In some examples, aspect ratios of maskregions can vary between one and six, or between one and four.

Positions of transmissive clusters can be selected to be semi-random sothat the clusters do not define a periodic grid but instead are randomlyoffset by an amount that is consistent with sensor element spacing,taking into consideration any CPB or light optical magnification ordemagnification. In one example, the random offset can correspond to amultiple of a sensor element dimension. As a specific example, anupper-left corner of region 1225 is offset from an upper-right corner ofregion 1221 by two sensor elements in both an X-direction and aY-direction. As another example, an upper-left corner of pixel 1223 isoffset from the upper-right corner of pixel 1225 by one sensor elementin the Y-direction and three sensor elements in an X-direction. Theoffsets can be selected so that the transmissive regions or clusters arenon-overlapping. In one embodiment, each of the transmissive regions isseparated by at least 5 μm.

Transmissive regions can be added to the mask pattern until the mask isfully populated. For example, the pixels can be added in a recursive oriterative manner until an end condition is met. As a specific example, afirst pixel can be selected in a corner (e.g., the upper-left corner) ofthe aperture mask. The first pixel can have a semi-random size, aspectratio, and/or orientation. Two additional pixels can be selected atoffsets from the first pixel. For example, a second pixel can beselected in a horizontal direction from the first pixel and a thirdpixel can be selected in a vertical direction from the first pixel. Thealgorithm can continue where each pixel spawns two more pixels, until anedge of the mask is reached and no more pixels can be added.

Aperture mask pattern 1100 was designed with more uniform pixels thanthe aperture mask pattern 1110. In particular, the aspect ratios of thepixels have a larger variance for the pattern 1110 than for the pattern1100. By clustering mask elements, a webbing separating mask elementsbecomes an active component of the mask. For example, the effectivetransmissivity can potentially be adjusted up to 50%. In one embodiment,a mask can be generated with mask element clustering where thetransmissive areas of the mask cover 40% to 50% of an active area of themask and have a feature size of 10 to 60 μm on a respective side. Inanother embodiment, a mask can be generated wherein the transmissiveregions cover between 30% and 50%, 40% and 50%, or 45% and 50% of maskactive area. Mask regions can have lengths and/or widths are between 5μm and 20 μm, 10 μm and 30 μm, or 5 μm and 50 μm or other dimensions.

Masks are typically defined by patterning a CPB absorbing layer on asubstrate, and then separating the patterned CPB absorbing layer fromthe substrate. In other examples, patterns can be defined on thinmembranes so that transmissive areas correspond to areas of the thinmembrane while non-transmissive areas are defined by a suitable materialsituated on the membrane. With such membrane-based masks, a patternedCPB absorbing layer need not be self-supporting.

Example 11 Single Sensor Systems

With reference to FIG. 13, a single sensor image system 1300 includes aradiation source that directs a beam 1302 to a mask 1304 that is coupledto a stage 1320. A stage controller 1322 is connected to the stage 1320to direct stage motions such as translations about one or more axes. (Apossible alternative arrangement of a stage coupled to a lens 1310 isalso shown.) The lens 1310 is situated to receive a mask modulated beam1308 from the mask 1304 and image the mask 1304 at a sample 1312. Insome examples, the mask 1304 and the sample 1312 are sufficiently closethat the lens 1310 is not used, typically dependent on the extent towhich the beam is collimated. A lens 1314 images the sample 1312 (asirradiated by a patterned beam from the mask 1312) so that beamradiation from a selected sample area (or an entire area of interest) isreceived by the single element sensor 1318 (or by a single element of asensor array). By acquiring a series of such sample area images withdiffering mask patterns (obtained by, for example, displacement of themask), an image of a specimen area can be obtained with a single elementsensor.

Example 12 Transmissive Imaging with Alternative Mask Placements

A representative transmissive imaging system 1400 including a radiationsource 1410 and a modulator 1430 is illustrated in FIG. 14. Theradiation source 1410 can generate electromagnetic radiation such asx-rays, gamma rays or light at infrared, near infrared, far infrared,and/or ultraviolet spectra wavelengths. As specific examples, theradiation source 1410 can generate electromagnetic waves withwavelengths in the ranges of 0.01-1 nm, 10-100 nm, 400-700 nm, or800-1200 nm, photon energies greater than 5 eV, 10 eV, 100 eV, 1 keV orgreater. The radiation source 1410 can also include a suitable opticalsystem for shaping and directing a radiation beam 1415. In someexamples, the radiation beam 1415 is collimated while in other examples,the radiation beam is focused or otherwise processed to have a selectedbeam divergence and beam size, suitable irradiating a target ofinterest.

The radiation beam 1415 can be transmitted through the sample 1420 andmodulated by one or more masks 1450, 1451 or by varying displacement ofthe sample 1420 with respect to one or more of the masks 1450, 1451 toform a mask-modulated image beam 1425. Variations in intensity, phase,wavelength, direction of propagation, or other characteristics in themask modulated image beam 1425 are associated with differences instructure and/or composition of the sample 1420. As shown in FIG. 14,the image can be spatiotemporally modulated using displacement of a beamwith respect to either of the masks 1450, 1451 or by movement of thesample 1420 with respect to either of the masks 1450, 1451. For sometypes of radiation, the mask modulated image beam 1425 can also bemodulated by moving or scanning the beam 1415. For example, for infraredradiation beams, beam scanning can be provided with a polygonal mirroror galvanometer based scanning system. In order to avoid or reduce beamdisplacements at a sensor array 1470, such scanning systems can beprovided both before and after the radiation beam 1415 interacts withthe sample 1420. As one typical example, the image beam is modulated bymechanically moving the mask 1450 using a translation or positioningstage 1455. The positioning stage 1455 can be coupled to the mask 1450that is scanned with respect to the image 1425 to produce amask-modulated image beam 1435. A sensor array 1470 receives themask-modulated image beam 1435 and integrates for a time durationestablished by a controller 1480, or otherwise determined. Acquisitionof the modulated image beam 1435 can be synchronized with the sensorarray 1470 to capture a single compressed image or multiple compressedimages in a video.

The mask 1450 applies a spatial encoding to the image beam 1425 based onan arrangement of mask areas that are associated with differingtransmittances, spectral transmittance, beam diffraction or deflection,phase, or other beam characteristics. In a convenient examples,differences in transmissivity are used and the mask 1450 includes areasassociated with at least first and second beam attenuations. Forconvenient description, such areas are referred to as transmissive andnon-transmissive. The pattern of the transmissive and non-transmissiveareas of the mask 1450 can be generated using the same methods as forgenerating the CPB masks. Alternatively, the mask 1450 can include atransmissive substrate with a patterned reflective or absorptive layer.As a specific example, the mask 1450 can be a chromium on glass mask.The glass substrate can be of suitable thickness so that it is rigid andsupports the mask. The chromium layer can be deposited on the glass byusing a lithographic process. In particular, the non-transmissive areasare associated with chromium or other metallic on the substrate andtransmissive areas are associated with areas that do not have suchmetallic coatings. For some types of radiation beams, suitabletransmissive substrates are available so that any layers that define avariable transmission pattern need not be self-supporting.Non-transmissive areas and/or transmissive areas can be discontinuous aswell in contrast to some examples of CPB masks in which a mask isdefined in a single metallic layer.

Example 13 Transmissive Diffraction Imaging

A representative transmissive diffraction imaging system 1500 isillustrated in FIG. 15. The system 1500 can include a beam source 1510configured to emit a beam 1515. A sample 1520 can be situated so that itis irradiated by the beam 1515 to produce an image beam 1525. The imagebeam 1525 can be focused by an objective lens 1540 to form an image. Theobjective lens also a diffraction pattern at a Fourier transform plane1545. The Fourier transform plane is referred to herein as a “transform”plane or a “diffraction” plane as well and is situated at a back focalplane of the objective lens 1540, the mask 1550 can be situated toreceive the focused image beam from the sample. A modulator 1530includes a mask 1550 that is situated at the transform plane 1545 so asto apply a mask modulation to a diffraction pattern associated with thesample 1520. In one example, the modulator 1530 can include adisplacement stage 1555 coupled to the mask 1550. The modulator 1530 canbe configured to produce a mask-modulated diffraction beam 1535 based onrelative displacements of the mask 1550 and the diffraction beam. Therelative displacements can be associated with a predefined periodic scanpattern that is controlled by a controller 1580. The modulateddiffraction beam 1535 can be focused by a projection lens 1560 to t atransform plane 1568 t a sensor array 1570 so that the sensor array 1570receives a modulated diffraction pattern. The sensor array 1570 can besynchronized with the modulator 1530 so that at a portion of theperiodic scan pattern is followed during an integration period of thesensor array 1570. The projection lens 1560 and the objective lens 1540can also be configured to direct a mask-modulated sample image to thesensor array 1570 instead of a mask-modulated diffraction pattern. Suchlens adjustment are particularly simple in CPB imaging in which controlof currents and or voltages applied to CPB lenses can be used to selectwhether a sample image or a sample diffraction pattern is directed tothe sensor array 1570.

Example 14 Additional Representative Mask

A representative mask 1600 and a magnified portion 1610 of the mask 1600are illustrated in FIG. 16. The mask 1600 is a single-pieceself-supported mask pattern. The non-transmissive area 1620 representedby the black areas is a constant width between the transmissive openings(such as the opening 1630). Preferably, different transmissive openingscan have different sizes, orientations, and aspect ratios.

Example 15 Additional Examples

The examples discussed above tend to emphasize the acquisition of videosequences using mask-modulation of image beams. In some application, thedisclosed methods and apparatus are arranged to acquire images or videosequences with reduce exposure to radiation beams. For example, thedisclosed methods can be applied to acquisition of electron beam imagesor image sequences with reduced electron beam exposure to avoid electronbeam induced damage. In some cases, the disclosed methods permit morerapid image data acquisition, with or without increasing spatial ortemporal resolution. As noted above, in one example, the disclosedmethods and apparatus permit increased temporal resolution so thatsample changes during exposure to electron beams or other radiation canbe investigated.

As one example, a method for transmission electron microscopy imagingincludes acquiring a series of temporally compressed images using acoded aperture or mask, and reconstructing the images using compressivesensing. The images can be coded and integrated into a single frameduring the acquisition. For example, the coded aperture or mask canenable the images to be compressed during an integration period of acamera. The camera can be a CCD camera or a direct detection camera. Themask can be positioned between a sample and the camera. The mask can bemoved during the integration period and the position and speed of themask can be determined by a function generator. For example, the maskcan be coupled to a piezoelectric element driven by the functiongenerator at a rate faster than the integration period of the camera.The time resolution of the images can be increased as the speed of thepiezoelectric element is increased. A translation range of the mask canincrease the number of images per second.

As another example, a system for transmission electron microscopyincludes a coded aperture or mask for producing a series of temporallycompressed images, and a piezoelectric element to move the mask.Movement of the mask can induce the temporal compression during a cameraintegration. The temporally compressed images can be reconstructed byusing a compressive sensing algorithm. The system can include a camera(such as a CCD camera or a direct detection camera), and the codedaperture or mask can enable the images to be compressed duringintegration on the camera. The mask can be positioned between a sampleand the camera. The piezoelectric element can be driven by a functiongenerator at a rate faster than an integration period of the camera. Theposition of the mask can be determined by the function generator.Increasing a translation range of the mask can increase the number oftemporally compressed images per second. The time resolution of theimages can be increased by increasing the speed of the piezoelectricelement. The function generator can also control reading and recordingof the system.

As another example, a method of transmission electron microscopyimaging, spectroscopy and/or spectrometry includes modulating a beam toinduce an encoded signal intended for compressive recovery andreconstructing images from the encoded signal. For example, the imagescan be reconstructed using a compressive sensing reconstructionalgorithm. The beam can be an electron beam, a neutron beam, an ionbeam, an x-ray beam, or an electromagnetic wave. The beam is modulatedeither before or after transmission through or reflection from a sample.The encoded signal is acquired either stochastically ordeterministically by one or more of: shifting of the image and/orspectrum location, size, contrast, focus, intensity, or selection ofspecific signal components. The modulation can be controlled by astationary aperture or a moving aperture so that the images are shiftedrelative to the aperture. In one embodiment, the images are shiftedrelative to the aperture by applying an electrostatic field to anelectron beam during generation or propagation in the microscope. Inanother embodiment, the images are shifted relative to the aperture bymoving the aperture using a piezoelectric stage. The aperture can be arandom or a deterministic coded aperture.

As another example, a system for transmission electron microscopyincludes an electron source for emitting an electron beam, and anelectrostatic field to induce shifting of image location, size,contrast, focus or intensity relative to an aperture or detector. Theimages are reconstructed, such as by using a compressive sensingreconstruction algorithm. The aperture can be a random or adeterministic coded aperture. The aperture can be stationary ormoveable.

As another example, a method for transmission electron microscopyimaging includes acquiring a series of temporally compressed imagesusing a coded aperture or mask, and reconstructing the images using acompressive sensing reconstruction algorithm. The images can be codedand integrated into a single frame during the acquisition of the images.For example, the coded aperture or mask can allow the images to becompressed during integration on a camera. The camera can be a CCDcamera or a direct detection camera. The mask can be positioned betweena sample and the camera or on/in the camera. The mask can move and theposition and speed of the mask can be determined by a functiongenerator. For example, the mask can be coupled to a piezoelectricelement driven by the function generator at a rate faster than thecamera. The time resolution of the images can be increased by the speedof the piezoelectric element. The translation range of the mask canincrease the number of images per second.

As another example, a system for transmission electron microscopyincludes a coded aperture or mask for producing a series of temporallycompressed images, and a piezoelectric element to move the mask. Themovement of the mask can induce the temporal compression during a cameraintegration. The images can be reconstructed by a compressive sensingalgorithm. The system can include a camera, and the coded aperture ormask can allow the images to be compressed during integration on thecamera. The mask can be positioned between a sample and the camera. Thepiezoelectric element can be driven by a function generator at a ratefaster than a frame time of the camera. The position of the mask can bedetermined by the function generator and a translation range of the maskcan determine the compression rate or number of images per-camera frame.For example, an increased translation range of the mask can increase thenumber of images per second. A framerate of recovered video can bedetermined by a translation range of the mask. The camera can be a CCDcamera or a direct detection camera. The function generator can controlreading and recording of the system.

Example 16 Example Computing Environment

FIG. 17 depicts a generalized example of a suitable computingenvironment 1700 in which the described innovations such as imageprocessing and mask pattern definition may be implemented. The computingenvironment 1700 is not intended to suggest any limitation as to scopeof use or functionality, as the innovations may be implemented indiverse general-purpose or special-purpose computing systems. Forexample, the computing environment 1700 can be any of a variety ofcomputing devices (e.g., desktop computer, laptop computer, servercomputer, tablet computer, etc.).

With reference to FIG. 17, the computing environment 1700 includes oneor more processing units 1710, 1715 and memory 1720, 1725. In FIG. 17,this basic configuration 1730 is included within a dashed line. Theprocessing units 1710, 1715 execute computer-executable instructions. Aprocessing unit can be a general-purpose central processing unit (CPU),processor in an application-specific integrated circuit (ASIC) or anyother type of processor. In a multi-processing system, multipleprocessing units execute computer-executable instructions to increaseprocessing power. For example, FIG. 17 shows a central processing unit1710 as well as a graphics processing unit or co-processing unit 1715.The tangible memory 1720, 1725 may be volatile memory (e.g., registers,cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory,etc.), or some combination of the two, accessible by the processingunit(s). The memory 1720, 1725 stores software 1780 implementing one ormore innovations described herein, in the form of computer-executableinstructions suitable for execution by the processing unit(s).

A computing system may have additional features. For example, thecomputing environment 1700 includes storage 1740, one or more inputdevices 1750, one or more output devices 1760, and one or morecommunication connections 1770. An interconnection mechanism (not shown)such as a bus, controller, or network interconnects the components ofthe computing environment 1700. Typically, operating system software(not shown) provides an operating environment for other softwareexecuting in the computing environment 1700, and coordinates activitiesof the components of the computing environment 1700.

The tangible storage 1740 may be removable or non-removable, andincludes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, orany other medium which can be used to store information in anon-transitory way and which can be accessed within the computingenvironment 1700. The storage 1740 stores instructions for the software1780 implementing one or more innovations described herein.

The input device(s) 1750 may be a touch input device such as a keyboard,mouse, pen, or trackball, a voice input device, a scanning device, oranother device that provides input to the computing environment 1700.The output device(s) 1760 may be a display, printer, speaker, CD-writer,or another device that provides output from the computing environment1700.

The communication connection(s) 1770 enable communication over acommunication medium to another computing entity. The communicationmedium conveys information such as computer-executable instructions,audio or video input or output, or other data in a modulated datasignal. A modulated data signal is a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia can use an electrical, optical, RF, or other carrier.

Example 17 Diffraction Pattern Imaging

With reference to FIG. 18, an imaging system 1800 includes a beam source1810 that directs a beam 1818 to a sample 1820. An objective lens 1840is situated to receive the beam 1818 from the sample 1818 and image thesample 1820 at a mask 1850 that is coupled to a stage 1855 and a stagecontroller 1880. A mask modulated beam 1835 from the mask 1850 isdirected to a projection lens 1860 and a sensor array 1870 is situatedin a transform plane associated with the projection lens 1860. Thetransform plane is conveniently a back focal plane of the projectionlens 1860.

Example 18 Diffraction Pattern Imaging

With reference to FIG. 19, an electron microscope system 1900 includesan electron beam source 1902 that directs an electron beam along an axis1901 to a condenser lens 1904. The condenser lens 1904 shapes theelectron beam and directs the electron beam to a sample 1906 that isimaged by a lens 1908 that forms a sample image at an image plane 1910.A second lens 1912 is situated along the axis 1901 to re-image thesample image. However, in the example of FIG. 19, a second image is notformed and instead a sensor array 1914 is situated at a back focal plane1916 of the second lens 1912 so that a Fourier transform or diffractionpattern associated with the sample 1906 is received by the sensor array1914. The beam received at the sensor array 1914 can be mask-modulatedwith a mask 1920 that provides a variable beam modulation based ondisplacements of the mask 1902 produced by a state 1955 as directed by acontroller 1980. A series of mask modulated sensor array signals can beprocessed as described above to produce a diffraction or Fouriertransform based image of the sample 1906. In some examples, a series ofmasks can be used, or the electron beam can be scanned to produce themodulated beam received by the sensor array 1914. Masks can also besituated at alternative locations so as to modulate the electron beamprior to the sample (e.g., mask 1921), in proximity to and downstream ofthe sample (e.g., mask 1920), or at or in proximity to an image plane(e.g., mask 1922). In addition, the sensor array can be situated atother locations such as at a back focal plan 1917 of the lens 1908 or ina so-called far field region.

The example of FIG. 19 is described with reference to electron beamsystems, but in other examples, other types of radiation are used. Forsome types of radiation, sensor arrays may not be readily available orpractical, and single sensors can be used instead to obtain suitabletransform plane data.

Example 19 Diffraction Images

Compressed image data can be obtained by using modulations based onvariations in focus, beam angle, image magnification, electrical ormagnetic deflection or other beam modulation, or for CPB systems, basedon interactions with adjacent CPBs. Compressive sensing approaches thatprovide phase retrieval can be used. The modulated (i.e., compressed)image data can then be used to reconstruct diffraction images thatinclude both phase and amplitude, so that images can be associated withphase, amplitude, or a combination of phase and amplitude.

Example 20 Mask-Less Diffraction Pattern Imaging

With reference to FIG. 20, an electron microscope system 2000 similar tothat of FIG. 19 includes an electron beam source 2002 that directs anelectron beam along an axis to a condenser lens 2004. The condenser lens2004 shapes the electron beam and directs the electron beam to a sample2006 that is imaged by a lens 2008 that forms a sample image at an imageplane 2010. A second lens 2012 is situated along the axis 2001 tore-image the sample image. However, in the example of FIG. 20, a secondimage is not formed and instead a sensor array 2014 is situated at aback focal plane 2016 of the second lens 2012 so that a Fouriertransform or diffraction pattern associated with the sample 2006 isreceived by the sensor array 2014. The beam received at the sensor array2014 can be modulated with by modulation of a beam current or other beamcharacteristic, lens focus (for example, of the lenses 2008, 2012 or asample location using controller 2080. A series of modulated sensorarray signals can be processed as described above to produce adiffraction or Fourier transform based image of the sample 2006. Theexample of FIG. 20 is described with reference to electron beam systems,but in other examples, other types of radiation are used. Amplitude,phase or combined amplitude and phase can be produced.

Alternatives and Variations

Any of the disclosed methods can be implemented as computer-executableinstructions or a computer program product stored on one or morecomputer-readable storage media (e.g., non-transitory computer-readablemedia, such as one or more optical media discs such as DVD or CD,volatile memory components (such as DRAM or SRAM), or nonvolatile memorycomponents (such as flash memory or hard drives)) and executed on acomputer (e.g., any commercially available computer, including smartphones or other mobile devices that include computing hardware). By wayof example and with reference to FIG. 17, computer-readable storagemedia include memory 1720, memory 1725, and/or storage 1740. The termcomputer-readable storage media does not include signals and carrierwaves. In addition, the term computer-readable storage media does notinclude communication connections (e.g., 1770).

Any of the computer-executable instructions for implementing thedisclosed techniques as well as any data created and used duringimplementation of the disclosed embodiments can be stored on one or morecomputer-readable storage media (e.g., non-transitory computer-readablemedia). The computer-executable instructions can be part of, forexample, a dedicated software application or a software application thatis accessed or downloaded via a web browser or other softwareapplication (such as a remote computing application). Such software canbe executed, for example, on a single local computer (e.g., any suitablecommercially available computer) or in a network environment (e.g., viathe Internet, a wide-area network, a local-area network, a client-servernetwork (such as a cloud computing network), or other such network)using one or more network computers.

For clarity, only certain selected aspects of the software-basedimplementations are described. Other details that are well known in theart are omitted. For example, it should be understood that the disclosedtechnology is not limited to any specific computer language or program.For instance, the disclosed technology can be implemented by softwarewritten in C++, Java, Perl, JavaScript, Adobe Flash, or any othersuitable programming language. Likewise, the disclosed technology is notlimited to any particular computer or type of hardware. Certain detailsof suitable computers and hardware are well known and need not be setforth in detail in this disclosure.

In view of the many possible embodiments to which the principles of thedisclosed technology may be applied, it should be recognized that theillustrated embodiments are only preferred examples and should not betaken as limiting the scope of the disclosure. We claim all that comeswithin the scope of the appended claims.

We claim:
 1. An imaging system, comprising: a modulator situated togenerate a modulated image beam of a sample; a sensor situated toreceive and detect the modulated image beam at a Fourier transformdiffraction plane; and a processor coupled to the sensor and operable toproduce a diffraction image of the sample based on a plurality ofdetected modulated image beams detected at the diffraction plane.
 2. Theimaging system of claim 1, wherein the processor is configured toproduce the diffraction image as a combined phase and magnitude image.3. The imaging system of claim 1, wherein the processor is configured toproduce the diffraction image as a phase image or a magnitude image. 4.The imaging system of claim 1, wherein the modulator is coupled tomodulate a beam focus, a beam displacement with respect to the sampleand/or the sensor, or a beam angle of incidence at the sample or thesensor, or an interference between beams.
 5. The imaging system of claim1, wherein the modulator includes at least one mask situated to receivea radiation beam and generate a mask-modulated image beam of the samplecorresponding to the modulated image beam and the processor is operableto produce the diffraction image of the sample based on a plurality ofdetected mask-modulated image beams detected at the diffraction plane.6. The imaging system of claim 5, wherein the at least one maskcomprises a plurality of transmissive and non-transmissive elementshaving a predetermined width and length, and wherein the modulator isoperable to produce a displacement of the radiation beam with respect tothe at least one mask by at least the predetermined width or thepredetermined length to produce the plurality of mask-modulated imagebeams.
 7. The imaging system of claim 6, wherein the at least one maskis situated to receive the radiation beam and generate themask-modulated image beam based on a relative displacement of the atleast one mask and the sample.
 8. The imaging system of claim 7, whereinthe at least one mask is situated to receive the radiation beam andgenerate the mask-modulated image beam based on the relativedisplacement of the radiation beam and the at least one mask that isproportional to a compression ratio.
 9. The imaging system of claim 8,further comprising a receptacle for retaining the sample, wherein the atleast one mask is positioned along a path of the radiation beam betweenthe receptacle and the sensor.
 10. The imaging system of claim 5,wherein the at least one mask comprises a plurality of masks, and theprocessor is operable to produce a diffraction image of the sample basedon a plurality of detected mask-modulated image beams detected at thediffraction plane and associated with the plurality of masks.
 11. Theimaging system of claim 1, further comprising a radiation beam sourcethat produces a radiation beam that is directed toward the sample,wherein the radiation beam is one of an X-ray beam, a gamma ray beam, aneutron beam, a charged particle beam (CPB), an infrared beam, and anultraviolet beam.
 12. The imaging system of claim 1, further comprising:a charged particle beam source situated to direct a charged particlebeam to the sample so as to produce a charged particle image beam thatcorresponds to the modulated image beam; and at least one maskpositioned along a charged particle beam path of the charged particlebeam between the charged particle beam source and the sample.
 13. Theimaging system of claim 12, wherein the modulator further comprises atranslation stage configured to displace the at least one mask withrespect to the charged particle image beam.
 14. The imaging system ofclaim 13, further comprising a piezoelectric positioner coupled to theat least one mask so as to displace the at least one mask along at leastone axis.
 15. The imaging system of claim 13, wherein the modulatorfurther comprises a beam deflector configured to scan the chargedparticle image beam with respect to the at least one mask.
 16. Theimaging system of claim 15, wherein the beam deflector is configured toscan the charged particle image beam with respect to the at least onemask in a predefined two-dimensional pattern.
 17. The imaging system ofclaim 1, further comprising a lens situated to receive the modulatedimage beam from the modulator, wherein the sensor is situated to receivethe modulated image beam from the lens.
 18. A method, comprising:modulating a radiation image beam to generate at least one modulatedimage beam; obtaining a plurality of modulated diffraction images bydetecting a plurality of modulated image beams with a sensor array at aback focal plane diffraction plane; and obtaining a diffraction image ofa sample by decompressing the plurality of modulated diffraction images.19. The method of claim 18, further comprising storing the diffractionimage in a computer readable medium.
 20. The method of claim 18, whereinthe modulation of the radiation image beam is produced by moving atleast one mask with respect to the radiation image beam.
 21. The methodof claim 20, wherein the at least one mask is periodically translatedusing a positioning stage to produce the modulated image beam.
 22. Themethod of claim 20, wherein the at least one mask comprises a pluralityof masks situated to produce the plurality of mask modulated image beamsthat are detected by the sensor array so as to obtain the plurality ofmask-modulated diffraction images.
 23. The method of claim 18, whereinthe radiation image beam is a charged particle beam (CPB) and themodulation of the radiation image beam is applied by moving theradiation image beam with an electrostatic or magnetic beam deflector.24. The method of claim 23, wherein the moving of the CPB is across atleast one mask in a predefined two-dimensional pattern.
 25. The methodof claim 18, wherein the modulating the radiation image beam to generatethe at least one modulated image beam is performed by modulating a beamfocus, beam current, beam position, sample position, or sample tilt. 26.The imaging system of claim 18, wherein the diffraction image is acombined phase and magnitude image.
 27. The imaging system of claim 18,wherein the diffraction image is a phase image or a magnitude image. 28.The method of claim 18, further comprising directing the plurality ofmodulated image beams to a lens, wherein the diffraction plane is adiffraction plane associated with the lens.
 29. A transmission electronbeam microscope, comprising: a beam source configured to emit anelectron beam; a sample receptacle for receiving a sample so as to beirradiated by the electron beam and produce an image beam; at least onemodulator situated to receive the electron beam from the beam source orthe image beam from the sample, the modulator operable to produce atleast one modulated image beam; and an imaging system situated toreceive the at least one modulated image beam at a diffraction plane,and produce a magnitude, phase, or combined magnitude and phasediffraction image of the sample.